1. Field of the Invention
The present invention relates to an image reading apparatus.
2. Description of the Related Art
As is known in the art, a typical image reading apparatus (or scanner) includes a light source for illuminating a document to be read out, and an array of light-receiving elements for detecting the light reflected on the document.
An image scanner is preferably provided with a “black level adjustment” function so that the output level of image reading signals can be adjusted. Specifically, it is known that the sensitivity of a light-receiving element (semiconductor device) tends to be varied under the influence of the temperature of the surrounding air. Thus, without taking any countermeasures, the level of an image reading signal outputted from a light-receiving element may unduly fluctuate during an image reading process. To eliminate this unfavorable fluctuation, the black level adjustment is performed in the following manner.
FIG. 4 of the accompanying drawings shows light-sensing IC chips 9 arranged side by side to detect the light reflected on a document to be read. (It should be noted here that the illustrated chips 9 are mere comparative examples for better understanding of the present invention, and that they should not be regarded as the prior art.) Each of the IC chips 9 includes an array of primary light-receiving elements 90 and one secondary (or “dummy”) light-receiving element 91. The dummy element 91 is covered with an aluminum film for shielding light.
Due to the aluminum covering, the level of a signal outputted from the dummy element 91 corresponds to the level of a signal obtained when reading out a black object. When the level of a signal from the dummy element 91 is unequal to the predetermined value (“zero” for example), it is shown that the current image reading condition is inappropriate (that is, the sensitivity of each light-receiving element is unduly altered due to the temperature of the surrounding air). In this case, the sensitivity of each light-receiving element needs to be adjusted for a proper image reading operation.
Though the IC chips 9 of FIG. 4 are arranged to perform the black level adjustment, they are disadvantageous in the following respects.
First, in each IC chip 9, a dummy light-receiving element 91 needs to be provided additionally to the primary elements 90 performing actual image reading. Unfavorably, this structure renders the fabrication of the IC chip 9 rather complicated, thereby resulting in an increase in production cost.
Second, the IC chip 9 needs room for provision of the dummy element 91. Thus, the width La of the chip 9 tends to become rather large.
Third, the chip 9 is different in design, due to the dummy element 91, from a light-sensing IC chip having no such dummy element. Thus, the manufacturer may need to fabricate two (or more) types of light-sensing IC chips to meet design requirements in different applications. Disadvantageously, this causes a production cost increase.